In recent years, a vehicle (hereinafter referred to as an □idling stop vehicle□) having an idling stop function for the purpose of, e.g., improvement of fuel consumption, has been in practical use, for example. Instantaneous start-up of an engine is required upon restart after idling stop. For this reason, a control system including a starter motor and a rotating electrical machine and configured to operate the starter motor and the rotating electrical machine in cooperation to start an engine has been proposed as an engine start-up device applied to the idling stop vehicle.
For example, Patent Literature 1 discloses the technique of changing a restart method depending on in which rotational speed range an engine rotational speed falls in a case where an engine is restarted upon satisfaction of predetermined restart conditions during an engine automatic stop period by an idling stop function. Specifically, the rotational speed range is divided into a first rotational speed range where the engine can be restarted only by resuming fuel injection, a second rotational speed range where the engine can be restarted only by a rotating electrical machine (a motor generator), and a third rotational speed range where cranking at an initial restart control stage by a starter motor is necessary. When the predetermined restart conditions are satisfied, a suitable restart method is selected based on in which rotational speed range of the first to third rotational speed ranges the engine rotational speed falls.